Fuel cell systems are configured to: supply a hydrogen-containing gas and an oxygen-containing gas to a fuel cell stack (hereinafter, simply referred to as a “fuel cell”) which is the main body of the system's power generating part; cause an electrochemical reaction between hydrogen and oxygen to progress; and extract chemical energy generated by the electrochemical reaction as electrical energy to generate electric power. Fuel cell systems are capable of generating electric power with high efficiency, and readily utilizing thermal energy that is generated during the power generation operation. Therefore, fuel cell systems are being developed as distributed power generation systems that make it possible to realize highly efficient energy utilization.
Generally speaking, it is often the case that an infrastructure for supplying the hydrogen-containing gas is not developed. Therefore, in many cases, conventional fuel cell systems are provided with a hydrogen generation apparatus. Such a hydrogen generation apparatus includes a reformer. The reformer causes a reforming reaction between a raw material and steam by using a Ru catalyst or Ni catalyst at temperatures of 600 to 700° C. The raw material is, for example, city gas (fuel gas) containing natural gas as a main component, or LPG, which is supplied from an existing infrastructure.
Generally speaking, a sulfur-containing odorant such as DMS, TBM, or THT is added to the raw material such as city gas or LPG supplied from an infrastructure for the purpose of facilitating the detection of leakage of the raw material. The raw material originally contains sulfur compounds. Such a sulfur-containing odorant and sulfur compounds (these are hereinafter collectively referred to as sulfur compounds) cause poisoning of a catalyst used in the reformer or the like, such as a Ru catalyst or Ni catalyst. As a result, the reforming reaction is hindered.
In order to prevent the poisoning caused by the sulfur compounds, the hydrogen generation apparatus usually includes a desulfurizer configured to remove the sulfur compounds from the raw material before the raw material is introduced into the reformer. One of such desulfurizers is a hydrodesulfurizer configured to add hydrogen to sulfur compounds at a high temperature by means of a catalyst for hydrogenation (hereinafter, referred to as a hydrogenation catalyst), thereby converting the sulfur compounds into hydrogen sulfide, and to remove the hydrogen sulfide through chemisorption (see Patent Literature 1, for example). Since the hydrodesulfurizer removes sulfur compounds by converting the sulfur compounds into hydrogen sulfide, the desulfurizing capacity of the hydrodesulfurizer is great and the desulfurizer can be made compact.
There are cases where oxygen is temporarily mixed into the raw material due to reasons of the infrastructure's side. In this respect, there is proposed a method of pre-reforming an oxygen-containing process gas, such as natural gas, peak shaving gas, LPG, etc. (see Patent Literature 2, for example).